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Context.IceCube observations point to active galactic nuclei (AGNs) as promising contributors to the observed astrophysical neutrino flux. Close to the central black hole, protons can be accelerated through magnetic reconnection to very high energies and subsequently interact with abundant X-ray photons in the source, leading to neutrino production. Aims.We investigate whether the diffuse neutrino flux observed by IceCube can originate, via proton acceleration, in reconnection-powered coronae of non-jetted AGNs. Methods.We created a library of neutrino spectral templates over a large grid of values for the three key model parameters: the proton plasma magnetization of the coronaσ_p, the X-ray coronal luminosity, and the black hole mass. Synchrotron cooling of pions and muons plays a significant role due to the large coronal magnetic fields. To infer the diffuse neutrino flux, we coupled the single-source model with a mock AGN catalog consistent with the observed X-ray and mid-infrared AGN samples at redshiftsz = 0 − 4. Results.The coronal emission satisfactorily explains the most recent IceCube measurements of the diffuse neutrino flux up to energies of ∼1 PeV, provided that ∼10% of the AGN coronae haveσ_p ∼ 10⁵, while the rest are distributed over a range of lower magnetizations. Coronal emission is suppressed at higher energies by pion and muon cooling so that another population is required, with jetted AGNs being strong candidates. 

Abstract Black holes (BHs) can acquire magnetic flux from their progenitor, through merger with a magnetized star, or via prolonged accretion. We study the evolution of BH magnetospheres using axisymmetric general relativistic magnetohydrodynamic simulations. We show that all initial magnetic field configurations, regardless of complexity, ultimately evolve into a split monopole. The magnetospheric evolution proceeds in two phases. In the first phase, pressure equilibrates on fast Alfvénic timescales (∼60 gravitational light-crossing times), and the flux on the BH event horizon drops sharply. The second phase proceeds in pressure balance, in which the magnetic flux decays via magnetic reconnection in current sheets, governing the dynamics on slower resistive timescales. We present an analytic model for the second phase, quantitatively describing the evolution of flux and current sheet dynamics. Additionally, we show that in the split monopole configuration, magnetic flux decays exponentially, with higher BH spin leading to slower decay. Our results enable a quantitative prediction of the timescales and spatial structure of these evolving magnetospheres, essential for understanding electromagnetic counterparts to gravitational wave events and reconnection-powered flares from accreting BHs. 

Abstract A ubiquitous feature of accreting black hole systems is their hard X-ray emission which is thought to be produced through Comptonization of soft photons by electrons and positrons in the vicinity of the black hole, in a region with optical depth of order unity. The origin and composition of this Comptonizing region, known as the corona, is a matter open for debate. In this paper we investigate the role of relativistic protons accelerated in black-hole magnetospheric current sheets for the pair enrichment and neutrino emission of AGN coronae. Our model has two free parameters, namely the proton plasma magnetizationσ_p, which controls the peak energy of the neutrino spectrum, and the Eddington ratio λ_X,Edd(defined as the ratio between X-ray luminosityL_Xand Eddington luminosityL_Edd), which controls the amount of energy transferred to secondary particles. For sources with λ_X,Edd≳ λ_Edd,crit(where λ_Edd,crit∼ 10^-1forσ_p= 10⁵or ∼ 10^-2forσ_p= 10⁷), proton-photon interactions andγγannihilation produce enough secondary pairs to achieve Thomson optical depthsτ_T∼ 0.1-10. In the opposite case of λ_X,Edd≲ λ_Edd,crit, the coronal pairs cannot originate only from hadronic interactions. Additionally, we find that the neutrino luminosity scales asL²_X/L_Eddfor λ_X,Edd≲ λ_Edd,crit, while it is proportional toL_Xfor higher λ_X,Eddvalues. We apply our model to four Seyfert galaxies, including NGC 1068, and discuss our results in light of recent IceCube observations. 

Abstract Recent advances in numerical simulations of magnetically arrested accretion onto supermassive black holes have shed light on the formation and dynamics of magnetospheric current sheets near the black hole horizon. By considering the pair magnetizationσ_ein the upstream region and the mass accretion rateṁ(in units of the Eddington mass accretion rate) as free parameters we estimate the strength of the magnetic field and develop analytical models, motivated by recent three-dimensional particle-in-cell simulations, to describe the populations of relativistic electrons and positrons (pairs) in the reconnection region.Applying our model to M87*, we numerically compute the non-thermal photon spectra for various values ofσ_e. We show that pairs that are accelerated up to the synchrotron radiation-limited energy while meandering across both sides of the current sheet, can produce MeV flares with luminosity of ∼ 10⁴¹ erg s^-1— independent ofσ_e— for a black hole accreting atṁ=10^-5. Pairs that are trapped in the transient current sheet can produce X-ray counterparts to the MeV flares, lasting about a day for current sheets with length of a few gravitational radii. We also show that the upstream plasma can be enriched due to photon-photon pair creation, and derive a new equilibrium magnetization ofσ_e∼ 10³-10⁴forṁ= 10^-6- 10^-5. Additionally, we explore the potential of magnetospheric current sheets to accelerate protons to ultra-high energies, finding that while acceleration to such energies is limited by various loss mechanisms, such as synchrotron and photopion losses from the non-thermal emission from pairs, maximal proton energies in the range of a few EeV are attainable in magnetospheric sheets forming around supermassive sub-Eddington accreting black holes. 

Abstract We study the magnetospheric evolution of a nonaccreting spinning black hole (BH) with an initially inclined split monopole magnetic field by means of 3D general relativistic magnetohydrodynamic simulations. This serves as a model for a neutron star (NS) collapse or a BH–NS merger remnant after the inherited magnetosphere has settled into a split monopole field creating a striped wind. We show that the initially inclined split monopolar current sheet aligns over time with the BH equatorial plane. The inclination angle evolves exponentially toward alignment, with an alignment timescale that is inversely proportional to the square of the BH angular velocity, where higher spin results in faster alignment. Furthermore, magnetic reconnection in the current sheet leads to exponential decay of event-horizon-penetrating magnetic flux with nearly the same timescale for all considered BH spins. In addition, we present relations for the BH mass and spin in terms of the period and alignment timescale of the striped wind. The explored scenario of a rotating, aligning, and reconnecting current sheet can potentially lead to multimessenger electromagnetic counterparts to a gravitational-wave event due to the acceleration of particles powering high-energy radiation, plasmoid mergers resulting in coherent radio signals, and pulsating emission due to the initial misalignment of the BH magnetosphere. 

Abstract Magnetic reconnection can power spectacular high-energy astrophysical phenomena by producing nonthermal energy distributions in highly magnetized regions around compact objects. By means of two-dimensional fully kinetic particle-in-cell (PIC) simulations, we investigate relativistic collisionless plasmoid-mediated reconnection in magnetically dominated pair plasmas with and without a guide field. In X-points, where diverging flows result in a nondiagonal thermal pressure tensor, a finite residence time for particles gives rise to a localized collisionless effective resistivity. Here, for the first time for relativistic reconnection in a fully developed plasmoid chain, we identify the mechanisms driving the nonideal electric field using a full Ohm law by means of a statistical analysis based on our PIC simulations. We show that the nonideal electric field is predominantly driven by gradients of nongyrotropic thermal pressures. We propose a kinetic physics motivated nonuniform effective resistivity model that is negligible on global scales and becomes significant only locally in X-points. It captures the properties of collisionless reconnection with the aim of mimicking its essentials in nonideal magnetohydrodynamic descriptions. This effective resistivity model provides a viable opportunity to design physically grounded global models for reconnection-powered high-energy emission. 

Abstract Supermassive black holes in active galactic nuclei are known to launch relativistic jets, which are observed across the entire electromagnetic spectrum and thought to be efficient particle accelerators. Their primary radiation mechanism for radio emission is polarized synchrotron emission produced by a population of nonthermal electrons. In this Letter, we present a global general relativistic magnetohydrodynamical (GRMHD) simulation of a magnetically arrested disk (MAD). After the simulation reaches the MAD state, we show that waves are continuously launched from the vicinity of the black hole and propagate along the interface between the jet and the wind. At this interface, a steep gradient in velocity is present between the mildly relativistic wind and the highly relativistic jet. The interface is, therefore, a shear layer, and due to the shear, the waves generate roll-ups that alter the magnetic field configuration and the shear layer geometry. We then perform polarized radiation transfer calculations of our GRMHD simulation and find signatures of the waves in both total intensity and linear polarization, effectively lowering the fully resolved polarization fraction. The telltale polarization signatures of the waves could be observable by future very long baseline interferometric observations, e.g., the next-generation Event Horizon Telescope. 

Abstract The origins of the various outbursts of hard X-rays from magnetars (highly magnetized neutron stars) are still unknown. We identify instabilities in relativistic magnetospheres that can explain a range of X-ray flare luminosities. Crustal surface motions can twist the magnetar magnetosphere by shifting the frozen-in footpoints of magnetic field lines in current-carrying flux bundles. Axisymmetric (2D) magnetospheres exhibit strong eruptive dynamics, i.e., catastrophic lateral instabilities triggered by a critical footpoint displacement of ψ crit ≳ π . In contrast, our new three-dimensional (3D) twist models with finite surface extension capture important non-axisymmetric dynamics of twisted force-free flux bundles in dipolar magnetospheres. Besides the well-established global eruption resulting (as in 2D) from lateral instabilities, such 3D structures can develop helical, kink-like dynamics, and dissipate energy locally (confined eruptions). Up to 25% of the induced twist energy is dissipated and available to power X-ray flares in powerful global eruptions, with most of our models showing an energy release in the range of the most common X-ray outbursts, ≲10 43 erg. Such events occur when significant energy builds up while deeply buried in the dipole magnetosphere. Less energetic outbursts likely precede powerful flares, due to intermittent instabilities and confined eruptions of a continuously twisting flux tube. Upon reaching a critical state, global eruptions produce the necessary Poynting-flux-dominated outflows required by models prescribing the fast radio burst production in the magnetar wind—for example, via relativistic magnetic reconnection or shocks. 

ABSTRACT Polarimetric measurements, especially if extended at high energy, are expected to provide important insights into the mechanisms underlying the acceleration of relativistic particles in jets. In a previous work, we have shown that the polarization of the synchrotron X-ray emission produced by highly energetic electrons accelerated by a mildly relativistic shock carries essential imprints of the geometry and the structure of the magnetic fields in the downstream region. Here, we present the extension of our analysis to the non-stationary case, especially suitable to model the highly variable emission of high-energy emitting BL Lacs. We anticipate a large ($$\Pi \approx 40{{\ \rm per\ cent}}$$), almost time-independent degree of polarization in the hard/medium X-ray band, a prediction soon testable with the upcoming mission IXPE. The situation in other bands, in particular in the optical, is more complex. A monotonic decrease of the optical degree of polarization is observed during the development of a flare. At later stages, Π reaches zero and then it starts to increase, recovering large values at late times. The instant at which Π = 0 is marked by a rotation of the polarization angle by 90°. However, at optical frequencies, it is likely that more than one region contribute to the observed emission, potentially making it difficult to detect the predicted behaviour. 

ABSTRACT Blazar emission models based on magnetic reconnection succeed in reproducing many observed spectral and temporal features, including the short-duration luminous flaring events. Plasmoids, a self-consistent by-product of the tearing instability in the reconnection layer, can be the main source of blazar emission. Kinetic simulations of relativistic reconnection have demonstrated that plasmoids are characterized by rough energy equipartition between their radiating particles and magnetic fields. This is the main reason behind the apparent shortcoming of plasmoid-dominated emission models to explain the observed Compton ratios of BL Lac objects. Here, we demonstrate that the radiative interactions among plasmoids, which have been neglected so far, can assist in alleviating this contradiction. We show that photons emitted by large, slow-moving plasmoids can be a potentially important source of soft photons to be then upscattered, via inverse Compton, by small fast-moving, neighbouring plasmoids. This interplasmoid Compton scattering process can naturally occur throughout the reconnection layer, imprinting itself as an increase in the observed Compton ratios from those short and luminous plasmoid-powered flares within BL Lac sources, while maintaining energy equipartition between radiating particles and magnetic fields.